The invention relates to a reflective object detecting apparatus which utilizes ultrasonic waves, and in particular, to such apparatus in which a plurality of detectors are selectively changed for purpose of detection.
In the field of vehicles, for example, an ultrasonic object detector is known which assists in guiding a driver by allowing him to be aware of an obstacle which is located out of the field of sight of the driver. Usually, such detector comprises an ultrasonic transmitter and an ultrasonic receiver which are disposed in close proximity to each other. Since the ultrasonic wave exhibits a sharp directivity, such detector is generally only capable of detecting objects located within a specified narrow range.
To overcome such limitation, there is proposed a detector which provides an increased spacing between the transmitter and the receiver to extend the range over which the detection is made possible, or an apparatus including a mechanical drive associated with a detector so that the directivity of the entire apparatus can be changed, thus extending the range over which the detection is possible.
However, where there is an increased spacing between the transmitter and receiver, the distance of transmission of the ultrasonic wave varies depending on the location or orientation of the object even though the actual distance between the detector and the object remains the same, causing an error of an increased magnitude in the detection of the distance. In addition, this arrangement does not permit the direction or orientation in which the object is located to be determined. Where a detectors is associated with the mechanical drive, the provision of the drive increases the size of the detector, rendering the mounting of the detector on the vehicle difficult. In addition, this arrangement requires a relatively long time to perform a mechanical scan, resulting in an increased length of time from the beginning of the detection until the termination of the detection. Since an object or objects located at short distances are to be detected for a vehicle, for example, an output of measurement or the response tends to be too slow unless the vehicle moves very slowly.
It will be appreciated that an ultrasonic detector requires a transducer which converts an electrical signal into an ultrasonic wave or converts an ultrasonic wave into an electrical signal. Most detectors which are available in the prior art are specially designed to allow only the detection, and hence there is no problem involved with the space requirement for the provision of the transducer.
However, for an obstacle detector which is designed for automobile use, the location may be limited depending on the construction of a transducer. By way of example, where conventional ultrasonic elements A, B and C are simply juxtaposed on a support member D as illustrated in FIG. 16, the transducer will have a thickness on the order of several centimeters and hence cannot be disposed on a lateral side of an automobile. If such disposition is allowed, there exists a protuberance, which degrades the appearance of the automibile. Obviously, if the transducer is initially designed to be embedded into a car body, the use of such thick detector on the side of the automobile is allowed. However, an obstacle detector of embedded type cannot be offered as an option, nor can its location be altered according to the preference of a user.